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Paraoxonase (PON1) and Detoxication of Nerve Agents
No full textParaoxonase (PON1) is a serum and liver enzyme that can hydrolyze in vitro a number of organophosphorus (OP) compounds, including the active metabolites of specific OP insecticides, and certain OP nerve agents such as sarin and soman. PON1 presents several genetic polymorphisms that influence its ability to hydrolyze OPs as well as its level of expression. Studies using animals, including transgenic mice, have shown that PON1 modulates the in vivo acute toxicity of certain OPs, particularly chlorpyrifos oxon and diazoxon. In contrast, because of its low catalytic efficiency toward paraoxon, PON1 does not influence the acute toxicity of this OP in vivo. The catalytic efficiency of PON1 toward nerve agents is similarly low, although some studies have shown that administration of exogenous PON1 can protect against the toxicity of soman and sarin. For use as catalytic bioscavengers, recombinant engineered PON1s need to be developed with an enhanced catalytic efficiency toward nerve agents. Such PON1s would be excellent candidates for prophylactic and therapeutic applications in case of OP poisoning. A parallel strategy would be that of identifying and studying agents that would increase levels of endogenous PON1
Paraoxonase protects against chlorpyrifos toxicity in mice
No full textParaoxonase can hydrolyze paraoxon (PO), chlorpyrifos-oxon (CPO) and other organophosphates. Previous studies have indicated that the levels of serum paraoxonase can influence the toxicity of PO and CPO. In the present study we have investigated whether exogenous paraoxonase administered to mice would offer protection toward the acute toxicity of a phosphorothioate, chlorpyrifos (CPS). Paraoxonase was purified from rabbit serum and injected i.v., or i.v. plus i.p., in mice. Inhibition of acetylcholinesterase (AChE) in brain, diaphragm, plasma and red blood cells was measured as an index of CPS (100 mg/kg) toxicity. Administration of paraoxonase 30 min before CPS increased plasma enzyme activity toward CPO by 35-fold, and protected against its toxicity; protection was still present at 24 h, when enzyme activity was still 20-fold over basal. When paraoxonase was given 30 min after CPS, a significant protection against CPS toxicity was still observed, while after 3 h the protective effect was decreased. To mimic conditions of severe acute poisoning, a higher dose of CPS (150 mg/kg) was also administered. Administration of paraoxonase 30 min after this exposure abolished cholinergic signs and significantly protected against AChE inhibition. These results indicate that exogenous paraoxonase offers significant protection against CPS toxicity when administered both before and after the organophosphate, suggesting that it may be considered as a potential additional treatment of organophosphate poisoning
Paraoxonase (PON1) gene in mice: sequencing, chromosomal localization and developmental expression
No full textSerum paraoxonase hydrolyses the toxic metabolites of several organophosphorus insecticides, as well as the nerve agents soman and sarin. We have previously shown that elevated serum paraoxonase levels protect mice against organophosphate toxicity. In the present study, we determined the cDNA sequence and chromosomal location of the mouse paraoxonase gene, as well as its developmental expression in mice and rats. The mouse cDNA encodes a protein of 355 amino acids and shows 81% identity with the human sequence. In situ hybridization demonstrated that the mouse paraoxonase gene maps to chromosome 6, a region conserved with the paraoxonase region of chromosome 7q21-22 in humans. Serum paraoxonase activities toward three substrates, paraoxon, chlorpyrifos-oxon and diazoxon, were very low at birth and increased with age reaching adult levels at 20 days in mice and 25 days in rats. The increase of serum paraoxonase activity in developing animals correlates well with the increased resistance to organophosphate poisoning that has been reported in previous studies
PARAOXONASE GENOTYPES, LIPOPROTEIN LIPASE ACTIVITY AND HIGH DENSITY LIPOPROTEINS
No full textParaxonase, an enzyme associated with the high density lipoprotein (HDL) particle, hydrolyzes paraoxon, the active metabolite of the insecticide parathion. Several studies have shown that paraxonase levels in humans have a distribution characteristic of two alleles, one with low activity and the other with high activity. Paraoxonase also has arylesterase activity, which does not exhibit activity polymorphism and can therefore serve as an estimate of enzyme protein. Although the ability of paraoxon to irreversibly inhibit lipoprotein lipase (LPL) has been exploited experimentally for many years, the role of plasma paraoxonase in lipoprotein metabolism is unknown. Seventy-two normal individuals were examined for paraoxonase genotypes, plasma paraoxonase and arylesterase activities, postheparin LPL and hepatic lipase (HL) activities, and lipoprotein levels to determine whether (1) paraoxonase activity or genotype determines lipoprotein levels via an effect on LPL or HL activity or (2) variation in LPL and HL activities determines HDL levels and indirectly affects paraoxonase activity and protein levels in plasma. In the entire group, paraoxonase activity was related to arylesterase activity and genotype. Whereas arylesterase activity was correlated with HDL cholesterol (HDL-C) and apolipoproteinA-I (apoA-I) levels, neither arylesterase nor paraoxonase was correlated with LPL or HL activity. Furthermore, LPL activity was positively correlated and HL inversely correlated with HDL cholesterol and apoA-I levels, whereas LPL was inversely correlated with triglyceride levels. The paraoxonase genotypes of the study group were 30 individuals homozygous for the low-activity allele, 38 heterozygotes, and 4 individuals homozygous for the high-activity allele. Paraoxonase genotype accounted for approximately .75 of the variation in paraoxonase activity. Paraoxonase activity was linearly related to arylesterase activity within each subgroup. No difference in either LPL or HL activity was seen as a function of paraoxonase genotype, nor were differences seen in plasma triglyceride or HDL-C by genotype by ANOVA. The relation between LPL and HL and components of HDL in the paraoxonase genotypic subgroups in general reflected the associations seen in the group as a whole. Multivariate analysis showed that LPL, HL, and arylesterase, a measure of paraoxonase mass, were independent predictors of HDL cholesterol, while paraoxonase genotype or activity was not. Thus, variation in LPL and HL appears to be significantly related to HDL cholesterol and apoA-I levels. The levels of HDL are a major correlate of paraoxonase protein levels, while paraoxonase genotype is the major predictor of plasma paraoxonase activity
Functional genomics of paraoxonase (PON1) polymorphisms: effects on pesticide sensitivity, cardiovascular disease and drug metabolism.
No full textThis review focuses on the functional genomics of the human paraoxonase (PON1) polymorphisms. Levels and genetic variability of the PON1 position 192 isoforms (Gln/Arg) influence sensitivity to specific insecticides or nerve agents and risk for. cardiovascular disease. A more recent area of investigation, the role of PON1 in drug metabolism, is also discussed. We emphasize the importance of considering both PON1 isoforms and PON1 levels in disease/sensitivity association studies
Examining the role of paraoxonase 2 in the dopaminergic system of the mouse brain
No full textBackground Paraoxonase 2 (PON2) is an intracellular antioxidant enzyme located at the inner mitochondrial membrane. Previous studies have found PON2 to be an important antioxidant in a variety of cellular systems, such as the cardiovascular and renal system. Recent work has also suggested that PON2 plays an important role in the central nervous system (CNS), as decreased PON2 expression in the CNS leads to higher oxidative stress and subsequent cell toxicity. However, the precise role of PON2 in the CNS is still largely unknown, and what role it may play in specific regions of the brain remains unexamined. Dopamine metabolism generates considerable oxidative stress and antioxidant function is critical to the survival of dopaminergic neurons, providing a potential mechanism for PON2 in the dopaminergic system. Methods In this study, we investigated the role of PON2 in the dopaminergic system of the mouse brain by comparing transcript and protein expression of dopaminergic-related genes in wildtype (WT) and PON2 deficient (PON2-def) mouse striatum, and exposing WT cultured primary neurons to dopamine receptor agonists. Results We found alterations in multiple key dopaminergic genes at the transcript level, however many of these changes were not observed at the protein level. In cultured neurons, PON2 mRNA and protein were increased upon exposure to quinpirole, a dopamine receptor 2/3 (DRD2/3) agonist, but not fenoldopam, a dopamine receptor 1/5 (DRD1/5) agonist, suggesting a receptor-specific role in dopamine signaling. Conclusions Our findings suggest PON2 deficiency significantly impacts the dopaminergic system at the transcript level and may play a role in mitigating oxidative stress in this system further downstream through dopamine receptor signaling
Paraoxonases-1, -2 and -3: What are their functions?
No full textParaoxonase-1 (PON1), an esterase/lactonase primarily associated with plasma high-density lipoprotein (HDL), was the first member of this family of enzymes to be characterized. Its name was derived from its ability to hydrolyze paraoxon, the toxic metabolite of the insecticide parathion. Related enzymes PON2 and PON3 were named from their evolutionary relationship with PON1. Mice with each PON gene knocked out were generated at UCLA and have been key for elucidating their roles in organophosphorus (OP) metabolism, cardiovascular disease, innate immunity, obesity, and cancer. PON1 status, determined with two-substrate analyses, reveals an individual's functional Q192R genotype and activity levels. The three-dimensional structure for a chimeric PON1 has been useful for understanding the structural properties of PON1 and for engineering PON1 as a catalytic scavenger of OP compounds. All three PONs hydrolyze microbial N-acyl homoserine lactone quorum sensing factors, quenching Pseudomonas aeruginosa's pathogenesis. All three PONs modulate oxidative stress and inflammation. PON2 is localized in the mitochondria and endoplasmic reticulum. PON2 has potent antioxidant properties and is found at 3- to 4-fold higher levels in females than males, providing increased protection against oxidative stress, as observed in primary cultures of neurons and astrocytes from female mice compared with male mice. The higher levels of PON2 in females may explain the lower frequency of neurological and cardiovascular diseases in females and the ability to identify males but not females with Parkinson's disease using a special PON1 status assay. Less is known about PON3; however, recent experiments with PON3 knockout mice show them to be susceptible to obesity, gallstone formation and atherosclerosis. Like PONs 1 and 2, PON3 also appears to modulate oxidative stress. It is localized in the endoplasmic reticulum, mitochondria and on HDL. Both PON2 and PON3 are upregulated in cancer, favoring tumor progression through mitochondrial protection against oxidative stress and apoptosis
Neurobehavioral assessment of mice following repeated postnatal exposure to chlorpyrifos-oxon
No full textChlorpyrifos (CPF), one of the most widely-used organophosphorus (OP) insecticides in agriculture, is degraded in the field to its oxon form, chlorpyrifos-oxon (CPO), which can represent a significant contaminant in exposures to adults and children. CPO is also responsible for the acetylcholinesterase (AChE) inhibition associated with CPF exposures; CPF is converted by liver CYP450 enzymes to CPO, which binds to and inhibits AChE and other serine active-site esterases, lipases and proteases. Young children represent a particularly susceptible population for exposure to CPF and CPO, in part because levels of the plasma enzyme, paraoxonase (PON1), which hydrolyzes CPO, are very low during early development. While a number of studies have demonstrated developmental neurotoxicity associated with CPF exposure, including effects at or below the threshold levels for AChE inhibition, it is unclear whether these effects were due directly to CPF or to its active metabolite, CPO. PON1 knockout (PON1(-/-)) mice, which lack PON1, represent a highly sensitive mouse model for toxicity associated with exposure to CPF or CPO. To examine the neurobehavioral consequences of CPO exposure during postnatal development, PON1(-/-) mice were exposed daily from PND 4 to PND 21 to CPO at 0.15, 0.18, or 0.25mg/kg/d. A neurobehavioral test battery did not reveal significant effects of CPO on early reflex development, motor coordination, pre-pulse inhibition of startle, startle amplitude, open field behavior, or learning and memory in the contextual fear conditioning, Morris water maze, or water radial-arm maze tests. However, body weight gain and startle latency were significantly affected by exposure to 0.25mg/kg/d CPO. Additionally, from PNDs 15-20 the mice exposed repeatedly to CPO at all three doses exhibited a dose-related transient hyperkinesis in the 20-min period following CPO administration, suggesting possible effects on catecholaminergic neurotransmission. Our previous study demonstrated wide-ranging effects of neonatal CPO exposure on gene expression in the brain and on brain AChE inhibition, and modulation of both of these effects by the PON1(Q192R) polymorphism. The current study indicates that the neurobehavioral consequences of these effects are more elusive, and suggests that alternative neurobehavioral tests might be warranted, such as tests of social interactions, age-dependent effects on learning and memory, or tests designed specifically to assess dopaminergic or noradrenergic functio
Paraoxonase polymorphisms and toxicity of organophosphates
No full textIn 1946, it was found that certain organophophorus (OP) insecticides could be enzymatically hydrolyzed by plasma (Mazur, 1946). Seminal studies by Aldridge (1953) indicated that A-esterases were capable of hydrolyzing OPs, whereas B-esterases [such as acetylcholinesterase (AChE)] reacted with a single OP molecule and were thus inhibited by this “suicide reaction”. Aldridge's proposal that an A-esterase hydrolyzed both phenylacetate and paraoxon was conclusively proven several decades later, when it was shown that recombinant paraoxonase/arylesterase catalyzed both activities (Gan et al., 1991). Studies in the late 1970s and early 1980s indicated that the plasma hydrolytic activity toward paraoxon was polymorphically distributed in human populations (Playfer, 1976; Eckerson et al., 1993; Mueller et al., 1983), suggesting a genetically based differential susceptibility to OP toxicity
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